Not Applicable
Not Applicable
The present invention relates to riser systems used in the offshore production of fluid hydrocarbons (e.g., petroleum and natural gas). More particularly, it relates to the field of steel catenary risers (SCRs), and specifically it relates to a system for attaching an SCR to a floating offshore facility or vessel, in which a substantially constant tension is applied to the SCR.
In the production of fluid hydrocarbons (xe2x80x9cproductxe2x80x9d) from undersea deposits, the movement of the product from the seabed to a surface facility, such as a floating production or storage platform or vessel, is critical. Typically, one or more conduits, or risers, are connected between a well-head or the like on the seabed and the surface facility.
Although the floating facility is typically moored or anchored, it is continuously exposed to a variety of forces from wind and water action that subject the facility to movements such as heave, roll, pitch, drift, and surge. Consequently, the riser system must be sufficiently compliant to compensate for such motion without experiencing undue stress and fatigue.
There have been a number of types of riser systems that have been developed to provide the requisite degree of compliance. One such system, as disclosed in, for example, U.S. Pat. No. 5,639,187xe2x80x94Mungall et al.; 6,257,801xe2x80x94Kelm et al.; and 5,957,074xe2x80x94de Baan et al., employs a plurality of rigid steel conduits that are laid between a subsea well or other fluid source on the seabed and a submerged buoy, the latter being tethered or moored to the seabed. The steel conduits are curved in a gentle catenary path between the seabed and the buoy, and are thus called xe2x80x9csteel catenary risersxe2x80x9d or xe2x80x9cSCRs.xe2x80x9d A plurality of flexible xe2x80x9cjumperxe2x80x9d conduits are then connected between the buoy and the surface facility to conduct fluid from the SCR to the facility. This approach necessitates the additional expense and time of deploying and anchoring the buoy. Such expense makes the submerged buoy arrangement not particularly cost effective, except perhaps in those systems employing a large number of risers.
Another approach, exemplified in the disclosure of U.S. Pat. No. 6,386,798xe2x80x94Finn, is to connect one or more SCRs directly to the surface facility, in a manner that allows the SCRs to move as the surface facility moves. One drawback with arrangements of this type, however, is that movement of the surface facility causes the touchdown point of the SCRs on the seabed to change. This is especially disadvantageous in relatively shallow water, where significant surface facility motions translate into large movements of the touchdown point along the seabed, thereby subjecting the SCRs to excessive fatigue, with consequent shortened fatigue life.
Consequently, there has been a long-felt need for an SCR system that provides for significant compliance of the riser system to compensate for substantial surface facility motion without the disadvantages attendant to the aforementioned prior art systems.
Broadly the present invention is an SCR system comprising a tensioning mechanism on a floating facility that controllably applies a substantially constant tension to an SCR that is fluidly coupled to the facility by a flexible jumper conduit. More specifically, the invention is a steel catenary riser (SCR) system for use with a floating facility on the surface of a body of water, comprising a tensioning device located on the floating facility, an SCR having an upper portion, connection means, connecting the upper portion of the SCR to the tensioning device, for controllably applying tension from the tensioning device to the SCR, and a flexible jumper conduit fluidly connected between the upper portion of the SCR and the floating facility for conducting fluid from the SCR to the floating facility. In a preferred embodiment, the upper portion of the SCR extends above the surface of the body of water; the connection means is attached to the upper portion of the SCR at an attachment point; and the flexible jumper conduit is fluidly coupled to the SCR near the attachment point.
As will be more fully appreciated from the detailed description that follows, by the application of a substantially constant tension to the SCR regardless of the relative motion between the SCR and the floating facility, the present invention provides improved fatigue life as compared with the fixed connection arrangements of the prior art, while being substantially more cost effective than the submerged buoy arrangement, especially for systems with a small number of risers.